1. Field of Invention
The present invention relates that a filter processing is performed in the input signal having the fluctuated intensity, and belonged to measurement of the chromatic dispersion distribution characteristics in the characteristics of the optical fiber used for ultrahigh speed optical communication field.
2. Description of the Prior Art
Presently, in the ultra high-speed optical communications field, in order to realize and maintain high quality communication, various researches about communication quality control and compensation technology of an optical fiber lines are advanced.
Based upon this, the demand from a market to characteristic evaluation of optical fiber became higher than former, and also in it, the chromatic dispersion characteristics attracts attention as an important item which discerns transmission restrictions by the wavelength band and the transmission speed.
And as for an example of this kind of chromatic dispersion distribution measurement of the optical fiber, Japanese Patent Publication No. Hei 10-83006 (corresponding to the U.S. Pat. No. 5,956,131 and the European Patent Application No. 0819926A2) is well known as shown in FIG. 6.
The principle of chromatic dispersion distribution measurement for the optical fiber is explained using FIG. 6, which showed a configuration of the conventional chromatic dispersion distribution measurement apparatus for the optical fiber.
In FIG. 6, a Laser Source 1 (LS1) generates coherent light having wavelength λ1, a Laser Source 2(LS2) generates coherent light having wavelength λ2, and these 2 lights are synthesized at a Coupler 3.
The synthesized light at the Coupler 3 is transformed to a pulse-like light synchronized to the clock signal (It is not illustrating) in A0 Switch 4, and amplified by an Erbium Doped Fiber Amplifier 5 (EDFA5).
An amplified light from EDFA 5 is supplied to a target Optical fiber 7 through an Optical Circulator 6.
Additionally, said Optical Circulator 6 branches the total backscattered light generated by incident light coming into the optical fiber 7.
A terminator 8 is repressing the Fresnel Reflection in the extreme of the optical fiber 7.
Besides, an Optical Band Pass Filter 9 operates to extract the one side of wavelength element of a four-wave mixing light generated by the interaction between each two wavelength in the total backscattered light generated by incident lights coming into the optical fiber 7.
An Optical Time Domain Reflectometer 10 (OTDR10) calculates a data pointing out the fluctuation of intensity based on a light of specific wavelength passing through the Optical Band Pass Filter 9 as a one side of wavelength element of four-wave mixing light generated by the interaction between a couple wavelengths of an incident light in the total backscattered light.
The data calculated in the OTDR10 accumulates to a RAM (Random Access Memory) of a Personal Computer 11 (PC11), and uses to a various computing.
FIG. 7 corresponding to FIG. 6 minutely shows a conventional procedure of measuring the chromatic dispersion distribution characteristics. Like FIG. 7, a target optical fiber connected to a measuring apparatus, and on starting the measurement of chromatic dispersion distribution characteristics for a target optical fiber;                Firstly, sets up measurement condition that two light sources having different wavelength each other (light source 1 & 2), OTDR and EDFA etc. (STEP S11).        After the setting up a measurement condition corresponding to STEP S11, executes measurement about a fluctuated intensity data of the light having a specific wavelength of the target optical fiber using OTDR. (STEP S12)        A data obtained in the measurement of STEP S12 send to a Personal Computer. (STEP S13)        A Personal Computer executes calculation of a chromatic dispersion distribution value using the data from OTDR. (STEP S14)        A chromatic dispersion distribution value, total dispersion value and a waive obtained by the processing in STEP S114 respectively are displayed in an indicator being inexistent in the figure. (STEP S15)        
In the conventional measurement of chromatic dispersion distribution showing in FIG. 6, the inputted signal to the personal computer PC11 disposed the filter processing, which is inexistent in the figure. And the suchlike filter processing installed externally a frequency pass-band being arbitrary value or fixed value.
Hereinafter, a configuration of the filter processing system of the conventional chromatic dispersion distribution measuring apparatus being implemented in the PC11 for example in FIG. 6 will be explained with reference to FIG. 2.
A filter processing of the conventional chromatic dispersion distribution apparatus comprises an input signal S1, a minimum chromatic dispersion value S24A, a maximum chromatic dispersion value S24B, a measuring condition parameter S25, a frequency converting section 21, a minimum frequency value S21A, a maximum frequency value 21B, filter coefficient generating section 22, filter coefficient S22, a filter processing section 23 and an output signal from the filter S23 (a signal passed through the filter).
Deriving a signal frequency from a minimum chromatic dispersion value S24A, a maximum chromatic dispersion value S24B and a measurement condition parameter S25 which are having a fixed value or an externally arranged arbitrary value, above mentioned frequency converting section 21 outputs a minimum frequency value S21A and a minimum frequency value 21B.
The filter coefficient generating section 22 generates the filter coefficient 22S using the minimum frequency value S21A and the maximum frequency value 21B assigned by the inputted pass range, and outputs them.
The filter processing section 23 using the filter coefficient S22 being assigned in the filter coefficient generating section 22, provides a filter function for the supplied input signal S1, and outputs an output signal from the filter S23 as the output. At that time, a kind of the filter depends on the filter coefficient generating section 22.
A processing flow of the conventional system shown in next FIG. 2 will be explained using a flowchart of FIG. 3.
For a start, a minimum chromatic dispersion value S24A, a maximum chromatic dispersion value S24B and the measuring condition parameter S25 are set and measurement is started.
Next, the minimum frequency value S21A and the maximum frequency value S21B are derived from the minimum chromatic dispersion value S24A, the maximum chromatic dispersion value S24B and the measuring condition parameter S25 at the frequency converting section 21.
Subsequently, the coefficient generating section 22 generates the filter coefficient S22 from the minimum frequency value S21A and the maximum frequency value S21B, which were calculated by the frequency converting section 21.
Moreover, the filter processing section 23 performs filter operation using the filter coefficient S22 provided from the coefficient generating section 22, and outputs an output signal from the filter S23.
After that, the chromatic dispersion distribution measurement results are accomplished by performing operation processing of the chromatic dispersion distribution to the output signal from the filter S23.
However, such a system described in the FIG. 2, there is a difficulty to providing consistently a best suited filter for the input signals having an intensity fluctuation, since the frequency pass-band of the filter is set to an arbitrary value from the outside or is fixed to a value derived from the frequency converting section 21.
A problem (goal) of the present invention is to provide the filter processing system configuring constantly optimum filter by setting up a frequency pass-band automatically.